In this study, thermodynamic properties, namely retention volume, infinitely dilute weight fraction activity coefficient, Flory-Huggins interaction parameter, solubility parameter of the polymer, partition coefficient and diffusion coefficients of the various solvents in poly(methyl methacrylate co butyl methacrylate) (PMMA co BMA) at infinite dilution of the solvent have been determined by inverse gas chromatography (IGC). In this technique, a small amount of the solvent was injected into the capillary column and its retention time was measured and used to calculate several polymer-solvent interaction parameters which are mentioned above. The solvents used in this study were methanol, ethanol, propanol, butanol, methyl acetate, ethyl acetate, propyl acetate, dichloromethane, trichloromethane, acetone, methyl methacrylate, butyl methacrylate for PMMA co BMA. The thermodynamic results, obtained from the experiments, indicated that trichloromethane and dichloromethane were the most suitable solvents among all the solvents studied for PMMA co BMA. The partition (K) and diffusion coefficients (D-p) of various solvents at infinite dilution of the solvent were calculated by using the model developed by Pawlisch et al. [C.A. Rawlisch, A. Macris, R.L. Laurence, Solute diffusion in polymers. I. The use of capillary column inverse gas chromatography, Macromolecules 20 (1987) 1564-1578]. The optimum K and Dp values that best fit the data were found and the model predicted experimental data very well. Vrentas-Duda free volume theory was used to correlate the diffusion data and to investigate the effect of solvent size on diffusion process. The theory has shown to correlate diffusion data above the glass transition temperature very well for the PMMA-co-BMA-solvent system. (c) 2005 Elsevier B.V. All rights reserved.